1. Field of the Invention
The invention is directed to a process for cold pilger rolling of thin-walled pipes having small outer diameters in which calibrated rolls are moved along the intermittently stationary pipe, inside of which a roll mandrel which is likewise calibrated assists in the shaping of the inner and outer pipe in a deformation or shaping region.
2. Description of the Prior Art
In cold pilger rolling of pipes, pipes having a large outer diameter are shaped into pipes with a smaller outer diameter. In so doing, calibrated rolls which are supported in a reciprocating roll stand are moved along the momentarily stationary pipe and deformed in a known manner when the groove rolls over the pipe. An internal tool, the roll mandrel, which is likewise calibrated ensures that the inner diameter and the pipe wall follow the desired regularity along the shaping distance.
The cold pilger rolling process has been known for a long time and has proven successful throughout the world as a process for producing pipes from various metals and metal alloys. Because of the particularly high quality of pipes produced by this process, there has been an increasing demand over recent years to produce small thin-walled precision pipes of high-quality steels and other high-strength materials by cold pilger rolling as well. Such precision pipes are conventionally produced by less cost-intensive shaping processes, e.g. drawing. Due to the steadily growing demand for quality, cold pilger rolling, at least as a final shaping step for these products, also has economic significance.
However, problems relating specifically to dimensions arise in cold pilger rolling of thin-walled pipes. Naturally, for very thin pipes the required internal tool, i.e. the roll mandrel, is correspondingly thin. The longitudinal forces and bending moments acting on the roll mandrel in cold pilger rolling therefore cause greater stress on the mandrel tool in these thin finished pipes and must be taken into account.
The influence of the effective forces is further increased when thin-walled pipes are to be produced. In this case, the deformation force applied by the rolls acts on the roll mandrel through the thin pipe wall. The thinner the pipe wall, the poorer the supporting action of the pipe. In the case of thick-walled pipes, this supporting action is capable of absorbing most of the shaping force applied by the rolls.
Due to these physical laws, the loading of the roll mandrel sets limits for the use of cold pilger rolling for the production of thin pipes and thin-walled pipes. Damage to the shaping tools, which cannot withstand the high forces, has been known to occur particularly at the start of the rolling process.